As Zn—Mg alloy plating has high corrosion resistance, a variety of products having a Zn—Mg alloy or single-layer or multi-layer Zn—Mg—X (X=Al, Ni, Cr, Pb, Cu, and the like) alloy including a third element have been developed. In addition, a variety of methods of manufacturing such products, such as hot dipping, vacuum deposition, alloying, and the like, have been used.
Recently, relatively many steel companies including Nisshin Steel (see Patent Document 1), POSCO (see Patent Document 2), Hyundai Hysco (see Patent Document 3), and the like have applied for patents related to Zn—Mg alloys.
However, in a case in which a Zn—Mg alloy film is formed using a conventional hot-dip process, when an Mg-added molten metal bath is exposed to air, a relatively large amount of dross may be formed therein due to oxidation of the Mg element, and in some cases, the molten metal bath may even ignite, which may be dangerous. For these reasons, a plating process may not be performed appropriately, or may be impossible to perform. Further, fumes generated from Mg, highly toxic to the human body, may pose a danger to steelworkers, as well as cause air pollution, and thus, the usage of Mg is significantly restricted.
In order to resolve such problems with a Zn—Mg alloy plating occurring when the hot-dip process is performed, a variety of patent applications providing a method of manufacturing a Zn—Mg alloy film using vacuum deposition (e.g. thermal evaporation, electron beam, sputtering, ion-plating, electromagnetic levitation physical vapor deposition, etc.) have been filed, but the methods disclosed in these patent applications merely provide methods of forming alloy films, without identifying the principle behind improved corrosion resistance of the Zn—Mg alloys disclosed therein. A common problem with such methods is that phases of such Zn—Mg alloys may include Mg2Zn11, MgZn2, MgZn, and Mg7Zn3, all intermetallic compounds having a higher brittleness than a brittleness of Zn or Mg. Thus, a relatively large amount of cracks or a peeling-off phenomenon may occur when a steel sheet is processed, and thus, the practical application of such steel sheets may be almost impossible. In particular, in a case in which an alloy heat treatment process is also performed, a Fe—Zn intermetallic compound may be formed due to counter diffusions of Fe and Zn at an interface between a steel sheet and Zn or a Zn—Mg alloy. Thus, adhesion thereof may be further degraded.
In addition, in a case in which an Mg content is decreased in a Zn—Mg alloy thin film in order to resolve the problem of degraded adhesion, corrosion resistance may not be significantly obtained, or as a size of a grain thereof is increased, the number of voids in the thin film may increase, and thus, a surface roughness thereof may be increased, and thus, various forms of spots may appear, and surface defects may occur as a surface color thereof is darkened.